The currently mainstream method for measuring interface states is a charge pumping method. The Chinese patent CN 101136347 discloses a measuring method for MOS transistor interface state. This measuring method comprises the following steps: step 1, obtaining a charge pumping current curve by the charge pumping measuring method, and obtaining another two charge pumping current curves by opening the drain terminal and the source terminal respectively; step 2, obtaining charge pumping currents at the source, drain, and channel by separating the identical and different portions of these three curves; step 3, obtaining the interface state densities at the source, drain, and channel from the charge pumping currents at the source, drain, and channel. However, as the critical dimension of the semiconductor device decreases continuously, the charge pumping method can not measure the charge pumping current accurately in the small device. This results from the small recombination current and the insufficient device accuracy. In addition, since current leakage is relatively large in a thin gate oxide, the influence of the gate oxide current leakage cannot be ignored in the measured bulk current, and this makes it difficult to measure accurately. Furthermore, as for the charge pumping method, the measured interface state density is an average value. In case of high-k dielectric material application, a large number of defects can be found not only in interface state, but also in the dielectric material, which significantly reduces the accuracy of the charge pumping method. Therefore, the conductance technique has attracted increasing attention in consideration of its accuracy.
The conductance technique is another method for measuring the interface state density. The basic principle is to measure the alternating current impedance Zm of a capacitor structure in the depletion region (the equivalent circuit is shown in FIG. 1a) with a measurement platform. FIG. 1b illustrates the equivalent circuit of the MOS capacitor in the depletion region, wherein Cox is the gate capacitance, and GT is the conductance corresponding to the path formed by electrons tunneling through the gate oxide, which generally can be ignored. GD is the capacitance of the depletion region, and Cit, Rit correspond to the impedance under a certain interface state density. FIG. 1c is a simplified circuit diagram of FIG. 1b, wherein Gp and Cp are equivalent impedance of the substrate. If GT is ignored, the equivalent circuit can be obtained as shown in FIG. 1d. There are two methods for characterizing interface state according to conductance spectra, i.e. a conductance-voltage technique and a conductance-frequency technique. The conductance-voltage technique is easy to operate and realize, but not accurate in theory. The conductance-frequency technique is accurate in theory. According to this theory, when the frequency of an alternating current signal coincides with the time constant of interface state at the Femi level, Gp/w has a maximum value.
In the conductance technique, the components connected in series of the structure to be measured may have a relatively large influence on the measurement results. Hence, in the method of converting the equivalent circuit as described above, the influence of such parasitic components can be modeled by incorporating certain parasitic elements in the circuit. Further, in the paper “A Methodology for Extraction of the Density of Interface States in the Presence of Frequency Dispersion via the Conductance Technique” on the IEEE transaction on electron devices by Sebastien et al. in 2010, it is proposed to modify the conductance technique, in which a fixed network of resistors and capacitors is used as an equivalent parasitic component for the structure to be measured. This method can effectively avoid the problem that the measured interface state density is an average interface state density in the conventional charge pumping method. In addition, in applications of high-k dielectric material, the interface state inside the high-k dielectric material can be measured. Therefore, this method is more accurate.
As mentioned above, during the actual measurement by the conductance technique, a series component such as a resistor in series with the device to be measured may have a remarkable influence on the measurement result. In the above mentioned paper, different models are used to eliminate these influences. As can be seen in the paper, the method is realized by incorporating an auxiliary device, and this auxiliary device is irrelevant with frequency.